Solvent dewaxing process for removing waxy constituents from hydrocarbon oils, especially lubricating oil stocks, are well known in the petroleum refining industry and a number of different processes have been developed. In all of them, the waxy oil is mixed with a solvent and the resulting mixture is chilled to a temperature at which the wax crystallizes out of solution. The amount of wax removed from the oil is dependent upon a number of factors including the type of oil, the amount and type of solvent and the temperature to which the mixture of oil and solvent is chilled.
Two types of solvent dewaxing process have become predominant in the industry. The first is the ketone dewaxing process which employs a ketone such as acetone, methyl ethyl ketone (MEK) or methyl isobutyl ketone as a solvent, either on its own or in combination with an aromatic solvent such as benzene, toluene or naphtha. The solvent is mixed with the oil after which the mixture is chilled using a scraped surface heat exchanger or, alternatively, mixing and chilling are accomplished simultaneously by injecting cold solvent into the oil at a number of points along a cooling tower through which the waxy oil is passing. Scraped surface heat exchangers may be used for additional cooling. A major disadvantage of all the ketone dewaxing processes is the need for scraped surface heat exchangers.
The other principal type of process in use is the autorefrigerant process in which a low molecular weight, volatile hydrocarbon such as propane, which is a gas at normal temperatures and pressures is used as the solvent. The autorefrigerative solvent is added to the waxy oil as a liquid, under pressure. It is then allowed to evaporate and in so doing cools the mixture, causing the wax to separate. The disadvantage of this process compared to the ketone processes is that the relatively high solubility of wax in the autorefrigerant at any given temperature does not permit the removal of as much wax as is achieved with the ketone dewaxing processes at the same filtration temperature. The pour point of the dewaxed oil is therefore higher for a given filtration temperature. This means that the oil must be chilled to substantially lower temperatures than in ketone dewaxing processes in order to achieve a specified wax content or pour point.
Dual solvent systems have also been proposed, for example, in U.S. Pat. No. 3,503,870, using a ketone as well as an autorefrigerant such as propane or propylene. The ketone has the effect of reducing the solubility of the wax in the autorefrigerant thereby avoiding one of the disadvantages of the autorefrigerant system and, in addition, the evaporative cooling provided by the autorefrigerant minimizes the reliance on scraped heat exchangers, thereby avoiding a major disadvantage of the ketone dewaxing system.
All these dewaxing processes have one common disadvantage, namely that they rely upon the use of a filter to separate the wax from the oil and the solvent. Although rotary filter systems have achieved a high degree of efficiency, permitting continuous operation, they are relatively expensive both in capital and running costs. They need to be large in order to deal with the desired throughput and this results in a substantial capital cost; the necessity of maintaining both vacuum and positive pressure on different parts of the filter represents an increment to operational costs which would otherwise be avoided. It would therefore be desirable to find some way of eliminating the filtering step from the dewaxing process.